1. Field of the Invention
The present invention relates to Coriolis gyros. More particularly, the invention pertains to an operating method for a Coriolis gyro and to suitable evaluation/control electronics.
2. Description of the Prior Art
Coriolis gyros (also referred to as vibration gyros) are in increasing use for navigation. They include a mass system that is caused to oscillate with the oscillation generally being a superimposition of a large number of individual oscillations.
The individual oscillations of the mass system are initially independent of one another, and may be regarded in abstract form as “resonators”. At least two resonators are required for operation of a vibration gyro; one (“first resonator”) is artificially stimulated to oscillate, with these oscillations being referred to below as a “stimulation oscillation”. The other (“second resonator”) is stimulated to oscillate only when the vibration gyro is moved/rotated. This is because Coriolis forces occur in this case that couple the first resonator to the second resonator, extract energy from the stimulation oscillation of the first resonator, and transmit it to the read oscillation of the second resonator. The oscillation of the second resonator is referred to below as the “read oscillation”.
In order to determine movements (in particular rotations) of the Coriolis gyro, the read oscillation is tapped off and the corresponding read signal (e.g., the read oscillation tapped-off signal) is investigated to determine whether any changes have occurred in the amplitude of the read oscillation as they represent a measure of rotation of the Coriolis gyro.
Coriolis gyros may be in the form of either an open-loop or a closed-loop system. In a closed-loop system, the amplitude of the read oscillation is continuously reset to a fixed value (preferably zero) by respective control loops. An example of a closed-loop version of a Coriolis gyro will be described below in conjunction with FIG. 2 to illustrate the method of operation of a Coriolis gyro. The gyro 1 includes a mass system 2 that can be caused to oscillate and is also referred to below as a “resonator”. (A distinction exists between this expression and the abstract “resonators” term previously employed for individual oscillations of the “real” resonator.) As mentioned, the resonator 2 may be considered as a system of two “resonators” (a first resonator 3 and a second resonator 4). The first and second resonators 3, 4 are coupled to a force transmitter (not shown) and to a tapping-off system (not shown). The noise produced by the force transmitter and the tapping-off systems is indicated schematically by Noise1 (reference symbol 5) and Noise2 (reference symbol 6).
The Coriolis gyro 1 includes four control loops. A first control loop controls the stimulation oscillation (i.e. the frequency of the first resonator 3) at a fixed frequency (resonant frequency). It comprises a first demodulator 7, a first low-pass filter 8, a frequency regulator 9, a voltage controlled oscillator (“VCO”) 10 and a first modulator 11.
A second control loop controls the stimulation oscillation at a constant amplitude. It comprises a second demodulator 12, a second low-pass filter 13 and an amplitude regulator 14.
Third and fourth control loops are employed to reset forces that stimulate the read oscillation. The third control loop includes a third demodulator 15, a third low-pass filter 16, a quadrature regulator 17 and a second modulator 18 while the fourth control loop comprises a fourth demodulator 19, a fourth low-pass filter 20, a rotation rate regulator 21 and a third modulator 22.
The first resonator 3 is stimulated at resonant frequency ω1. The resultant stimulation oscillation is tapped off, demodulated in phase by means of the first demodulator 7, and a demodulated signal component is supplied to the first low-pass filter 8 that removes the sum frequencies. (The tapped-off signal is also referred to below as the stimulation oscillation tapped-off signal.) An output signal from the first low-pass filter 8 is applied to a frequency regulator 9 which controls the VCO 10, as a function of the signal supplied to it, such that the in-phase component essentially tends to zero. The VCO 10 passes a signal to the first modulator 11, which controls a force transmitter such that the first resonator 3 has a stimulation force applied to it. When the in-phase component is zero, the first resonator 3 oscillates at its resonant frequency ω1. (It should be noted that all of the modulators and demodulators are operated on the basis of this resonant frequency ω1.)
The stimulation oscillation tapped-off signal is also supplied to the second control loop and demodulated by the second demodulator 12. The output of the second demodulator 12 is passed to the second low-pass filter 13, whose output signal is, in turn, applied to the amplitude regulator 14. The amplitude regulator 14 controls the first modulator 11 in response to this signal and the output of a nominal amplitude transmitter 23 to cause the first resonator 3 to oscillate at a constant amplitude (i.e. the stimulation oscillation has constant amplitude).
As mentioned above, movement/rotation of the Coriolis gyro 1 results in Coriolis forces (indicated by the term FC·cos(ω1·t) in the drawing) that couple the first resonator 3 to the second resonator 4, stimulating the second resonator 4 to oscillate. A resultant read oscillation of frequency ω1 is tapped off, and a corresponding read oscillation tapped-off signal (read signal) is supplied to both the third and fourth control loops. This signal is demodulated in the third control loop by the third demodulator 15. Sum frequencies are removed by the third low-pass filter 16 and the low-pass-filtered signal is supplied to the quadrature regulator 17. The output of the regulator 17 is applied to the third modulator 22 so that corresponding quadrature components of the read oscillation are reset. Analogously, the read oscillation tapped-off signal is demodulated by the fourth demodulator 19 in the fourth control loop. It passes through the fourth low-pass filter 20, and the low-pass-filtered signal is applied to the rotation rate regulator 21 (whose output signal is proportional to the instantaneous rotation rate) and passed as the rotation rate measurement result to a rotation rate output 24. On the other hand, it is passed to the second modulator 18, which resets the corresponding rotation rate components of the read oscillation.
A Coriolis gyro 1 as described above may be operated in both double-resonant and non-double-resonant versions. When operated in a double-resonant form, the frequency ω2 of the read oscillation is approximately equal to the frequency ω1 of the stimulation oscillation. In contrast, in the non-double-resonant case, the frequency ω2 of the read oscillation differs from the frequency ω1 of the stimulation oscillation. In double resonance, the output signal from the fourth low-pass filter 20 contains corresponding information about the rotation rate. In the non-double-resonant case, in contrast, the output signal from the third low-pass filter 16 contains this information. A doubling switch 25 is provided to switch between the different operating modes of double resonance/non-double resonance. The switch 25 selectively connects the outputs of the third and of the fourth low-pass filter 16, 20, selectively to the rotation rate regulator 21 and to the quadrature regulator 17.
The design of a Coriolis gyro as described above, in particular that of the evaluation/control electronics, offers the advantages of relatively high rotation rate sensitivity coupled with the simple mechanical structure of the resonator 2. A disadvantage of the arrangement is its high complexity in terms of the electronic components of the evaluation/control electronics. A plurality of digital/analog converters is required in the embodiment of the Coriolis gyro of FIG. 2 (e.g., at the points 26, 27 and 28). They are expensive and require a large amount of electrical power. The digital/analog converters frequently require a number of supply voltages and are difficult to integrate with other electronic components (in particular, digital components) restricting miniaturization. Furthermore, at least two analog/digital converters must be used in the embodiment of the Coriolis gyro shown in FIG. 2 (at the points 291 and 292).